E. coli bacteria was detected in a storage tank in the Pierce Hill Reservoir Thursday morning, said Hans Larsen, executive director of Wellesley’s Board of Selectmen.

Officials are awaiting results of tests on the town’s water supply to determine how severe the contamination is and when the water will be safe for consumption.

Larsen said the Department of Environmental Protection and the Department of Public Health need multiple rounds of negative test results before they can deem the water safe for consumption.

No test results were available by late Friday afternoon.

“We recognize this is a significant inconvenience for the town, and we are anxious to resolve this issue,” Larsen said.

Michael Pakstis, director of the town’s Department of Public Works, told the Globe Thursday that testing will be a challenge because the tank where the contamination was discovered is underground, so “investigating it is a little more difficult.”

Pakstis said the town’s water supply is tested frequently, and it is unclear what exactly led to the contamination.

He said that this is the first time in decades such contamination had been detected in Wellesley.